Better error message for @testsetup missing module

[nickrobinson251]

this typo lead to an unhelpful error message:

  # foo.jl
  @testsetup FooSetup   # <-- should be `@testsetup module FooSetup`
      x = 1
  end

julia> using ReTestItems

julia> include("foo.jl")
ERROR: LoadError: type Symbol has no field head
Stacktrace:
 [1] getproperty(x::Symbol, f::Symbol)
   @ Base ./Base.jl:38
 [2] var"@testsetup"(__source__::LineNumberNode, __module__::Module, mod::Any)
   @ ReTestItems ~/repos/ReTestItems.jl/src/macros.jl:51
 [3] include(fname::String)
   @ Base.MainInclude ./client.jl:476
 [4] top-level scope
   @ REPL[9]:1
in expression starting at /Users/nickr/repos/ReTestItems.jl/foo.jl:1